1. Field of the Invention
This invention relates in general to drill bits and in particular to roof bits used to drill holes in mine roofs for the purpose of installing anchor bolts to support mine roofs during mining operations.
2. Description of the Prior Arts
In the early 1950's, the coal mining industry began to support mine roofs with an anchor bolting system, rather than by the former system of timbering. During installation of the anchor bolting system, one inch to one and one-half inch holes are drilled in the roof of the mine. Six-inch square plates are then bolted to the roof with roof bolts that extend upward through several layers of rock. The bolts and plates bind the strata together so that the bottom layers cannot collapse into the mine. Two different methods are used to secure the bolts in the drilled holes. One method is to use mechanical expansion and tensioning bolts, and the other method is to cement the bolts in place with a setting resin.
The holes for the bolts are generally drilled with a drilling system having a power source rotating a length of drill steel with a roof bit attached at the end. An example of such a drilling system is shown in U.S. Pat. No. 4,099,585. Early drills were light-weight and high-speed, with little upward thrust, but the power sources on more modern drills are usually around 75 horsepower and can generate up to approximately 10,000 pounds of thrust and 350 pounds torque. Rotation speeds are variable between 100 and 700 rpm, but approximately 500 rpm is most common.
As a hole is drilled, the dust that is produced must be removed from the hole in some manner. Early drilling systems either had augers to pull the dust out or merely allowed the dust to fall out of the hole. Later it was discovered that a vacuum could be put on the hole to draw the dust out between the drill steel and the sides of the hole. In modern drilling systems the dust is forced by a vacuum through dust ports into the interior of a hollow drill steel, and then out of the hole. U.S. Pat. No. 3,187,825 illustrates both interior and exterior vacuum dust collection systems.
The modern roof bit has a hard carbide insert which forms the cutting edge of the bit. As the roof of the mine is drilled, particles of dust are directed downward, toward the dust ports, by multiple work surfaces on the bit body. Many different combinations of work surfaces and dust ports have been tried in the attempt to maximize efficiency and minimize costs. For example, U.S. Pat. Nos. 3,434,553 and 3,434,554 show roof bits in which the dust ports are drilled in the dust control surfaces downwardly at an angle. In contrast, U.S. Pat. No. 4,313,506 discloses a roof bit in which the dust ports are positioned below the work surfaces.